The plane's crew has apparently now explained to authorities that they became distracted during a "heated discussion over airline policy" and lost track of their location. Right. That's like a teenage girl getting into a car accident and then explaining to her father, "I'm so sorry Daddy. I just got so caught up bragging to my friends about what a great daddy you are that I got a little distracted for a minute..." According to CNN, the FAA is investigating whether the pilots may have fallen asleep.
I suppose that the cockpit audio recordings in the plane's "black box" will quickly reveal what really happened. But neither of those two relatively straightforward explanations sound entirely correct to me. Maybe they were talking about basketball, or gladiator films: